Method for producing a multilayer coating and the use thereof

ABSTRACT

A process for producing a multicoat system on a substrate, in which a) an electrodeposition coating film is deposited on the substrate, b) the electrodeposition coating film is predried by heating to a predrying temperature for a predetermined period, c) a coat of a surfacer is applied to the electrodeposition coating film, and d) the electrodeposition coating film and the coat of the surfacer are baked together at elevated temperatures, and in which the predrying temperature in step b) is equal to the temperature (T p ) or lies above the temperature (T p ) at which the loss factor tanδ, which is the quotient formed from the loss modulus E″ and the storage modulus E′, of the unbaked electrodeposition coating material shows a maximum, and the use of the resulting multicoat system.

[0001] The invention relates to a process for producing a multicoatsystem on an electrically conducting substrate, in which anelectrodeposition coating film is deposited on the substrate, theelectrodeposition coating film is predried by heating to a predryingtemperature, a coat of a surfacer is applied to the electrodepositioncoating film, and the electrodeposition coating film and the coat of thesurfacer are baked together at elevated temperatures, and the use of themulticoat systems obtained in this way.

[0002] The invention further relates to a method of determining thepredrying temperature of the electrodeposition coating material in aprocess of the abovementioned variety by means of dynamic mechanicalthermoanalysis (DMTA). DMTA is known, for example, from the GermanPatent Application DE 44 09 715 A1, where it is used for quantitativedescription of the chemical crosslinking reactions in coating filmsdeposited on strips of fabric having a defined profile of mechanicalproperties. By using electrically conductive strips of material it isalso possible to deposit and investigate electrodeposition coatingmaterials. Determination of the predrying temperature of theelectrodeposition coating films by means of DMTA is not described in DE44 09 715 A1.

[0003] For producing multicoat systems having a primer comprising anelectrodeposition coating material with a coat situated above it, theprocess of what is known as wet-on-wet application of electrodepositioncoating material and at least one further coat is known, for example,from the patent applications EP 0 817 684 A1, EP 0 639 660 A1, EP 0 595186 A1, EP 0 646 420 A1 or DE 41 26 476 A1. The coating materials whichare applied wet-on-wet may be in liquid (aqueous, conventional or powderslurry) or powder form. The coating materials may be pigmented andunpigmented and may be used to produce surfacers or functional coats(pigmented) or clearcoats (unpigmented), but especially to producesurfacers.

[0004] During the implementation of the wet-on-wet processes, theapplied electrodeposition coating film is generally predried prior tothe application of the next coating material. This is generally doneunder conditions in which water and solvents are largely evaporated fromthe electrodeposition coating film. This procedure is environmentallyand economically advantageous and, moreover, generally producesbetter-quality coatings.

[0005] Nevertheless, it is possible again and again to observe problemswith the surface appearance (i.e., appearance of the overall systemincluding clearcoat). These problems are manifested, for example, in thevalues of a longwave/shortwave wavescan (light reflection) which gives avalue for the amount of scattered light. The flow of the coatedmaterial, as well, in many cases fails to meet requirements.

[0006] Attempts have been made to solve these problems, in a very widevariety of ways.

[0007] For example, in the process according to the German PatentApplication DE 41 26 476 A1, the use of electrodeposition coatingmaterials is restricted to those which on curing have a baking loss ofless than 10%. However, this imposes severe restrictions on the user inthe selection of suitable electrodeposition coating materials.

[0008] The process according to the European Patent Application EP 0 646420 A1 uses electrodeposition coating materials and powder coatingmaterials whose baking temperatures are harmonized with one another.Thus, the interval of the minimum baking temperature of the second coat(powder coat) should lie above the interval of the first coat(electrodeposition coat), or the intervals should overlap such that thelower limit of the interval of the minimum baking temperature of thesecond coat lies above the lower limit of the interval of theelectrodeposition coat. In other words, the electrodeposition coatingmaterial has a baking temperature which is lower than the bakingtemperature of the powder coating material. Despite this adaptation ofthe baking temperatures, problems of appearance and of flow continue tooccur. Moreover, extensive flaking may occur on stone impact.

[0009] Accordingly, the attempts to solve the problems stated haveessentially concentrated on selecting only electrodeposition coatingmaterials having a low volume shrinkage or on adapting to one anotherthe baking temperatures of the electrodeposition coating film and thesecond coating film.

[0010] It is an object of the present invention to find a new process ofthe variety mentioned at the outset for producing multicoat systems onelectrically conductive substrates which no longer has the disadvantagesof the prior art but which instead, in an environmentally andeconomically efficient manner, produces high-quality multicoat systemswhich have an improved surface appearance (appearance of the overallsystem including clearcoat) and better flow of the coating. The improvedappearance should be manifested significantly in particular in thevalues of a longwave/shortwave wavescan (light reflection) which gives avalue for the amount of scattered light. Moreover, the antistonechipproperties should be improved. A further aspect of the present inventionis the use of the multicoat systems in automobile coating and inindustrial coating.

[0011] This object is achieved by means of a process for producing amulticoat system on a substrate or the use of this multicoat system, inwhich

[0012] a) an electrodeposition coating film is deposited on thesubstrate,

[0013] b) the electrodeposition coating film is predried by heating to apredrying temperature of the electrodeposition coating material for apredetermined period,

[0014] c) a coat of a surfacer is applied to the electrodepositioncoating film, and

[0015] d) the electrodeposition coating film and the coat of thesurfacer are baked together at elevated temperatures.

[0016] A feature of the process is that the predrying temperature liesat or above, preferably from 0° C. to 35° C. and more preferably from 5°C. to 25° C. above, the temperature T_(p) at which the loss factor tanδof the unbaked (i.e., uncrosslinked) electrodeposition coating materialshows a maximum.

[0017] The recoverable energy component (elastic component) in thedeformation of a viscoelastic material such as a polymer is determinedby the size of the storage modulus E′, whereas the energy componentconsumed (dissipated) in this process is described by the size of theloss modulus E″. The moduli E′ and E″ are dependent on the rate ofdeformation and the temperature. The loss factor tanδ is defined as thequotient formed from the loss modulus E″ and the storage modulus E′.tanδ may be determined with the aid of dynamic mechanical thermoanalysis(DMTA) and represents a measure of the relationship between the elasticand plastic properties of the electrodeposition coating film (Th. Frey,K. -H. Groβe-Brinkhause, U. Röckrath: Cure Monitoring of ThermosetCoatings, Progress in Organic Coatings 27 (1996) 59-66).

[0018] It has surprisingly been found that the achievement or exceedanceof the above-mentioned temperature T_(p) is critical to the success ofwet-on-wet application and that the evaporation of water or solvents isof little or no priority. Processes whose predrying operation is aimedonly at removing solvents, such as drying with predried air at reducedtemperatures, for example, therefore generally give results which aremuch poorer.

[0019] When the predrying temperature of the invention is kept to, animproved surface appearance may be observed (appearance of the overallsystem including clearcoat). This is manifested, for example, in thevalues of a longwave/shortwave wavescan (light reflection) which gives avalue for the amount of scattered light. The flow of the coatingmaterial is also improved.

[0020] In addition, it is possible to observe an improvement in theantistonechip properties. In particular, the area of flaking is smallerand there is better adhesion to the substrate.

[0021] The reasons for the decisive influence of the predryingtemperature attained are unelucidated. It is possible that relaxationprocesses take place within the electrodeposition coating film (cf.Encyclopedia of Polymer Science and Engineering, Vol. 5, John Wiley andSons, pages 299-329). The factor involved need not necessarily includeglass transitions, since with certain electrodeposition coatingmaterials it was possible in experiments to rule out explicitly such aglass transition within the temperature range of the predrying.

[0022] In the process of the invention, it is possible in many cases toprescribe lower predrying temperatures than in the case of the priorart. In these cases, therefore, it is not necessary to cool down thesystem so greatly before applying the surfacer, so simplifying theprocess, saving energy, and leading to lower capital costs and operatingcosts.

[0023] It is a particular advantage of the process of the invention thatthe optimum predrying temperature—irrespective of whether it iscomparatively high or comparatively low—may be determined in a simplemanner for a given electrodeposition coating material.

[0024] The prescribed period for the implementation of predrying in stepb) is typically from 1 to 60 minutes, preferably from 5 to 15 minutes.After predrying, the substrate is preferably taken back to the ambienttemperature before the surfacer is applied. The time between theapplication of the electrodeposition coating material and theapplication of the surfacer is arbitrary.

[0025] In accordance with one development of the process of theinvention, the coat of a surfacer applied in step c) is predried forfrom about 1 to 30 minutes, preferably for from 10 to 20 minutes, beforethe conjoint baking in step d). This predrying takes place at atemperature which is dependent on the surfacer material, so that theskilled worker is readily able to determine the optimum temperature onthe basis of his or her general knowledge in the art, possibly with theaid of rangefinding tests.

[0026] The thickness of the fully cured electrodeposition coating filmis preferably from 10 μm to 30 μm, with particular preference from 15 μmto 20 μm. The thickness of the fully cured surfacer coat depends on thesurfacer material and is preferably from 10 μm to 60 μm.

[0027] Suitable baths for the electrodeposition coating operation areall customary anodic or cathodic electrodeposition coating baths.

[0028] These electrodeposition coating baths are aqueous coatingmaterials having a solids content of in particular from 5 to 30% byweight.

[0029] The solids of the electrodeposition coating material comprise

[0030] (A) customary and known binders which carry functional groups(a1) which are ionic or are convertible into ionic groups, andfunctional groups (a2) capable of chemical crosslinking, these groupsbeing externally crosslinking and/or self-crosslinking, but especiallyexternally crosslinking;

[0031] (B) if desired, crosslinking agents which carry complementaryfunctional groups (b1) which are able to enter into chemicalcrosslinking reactions with the functional groups (a2), and which areemployed mandatorily when the binders (A) are externally crosslinking;and

[0032] (C) customary and known coatings additives.

[0033] Where the crosslinking agents (B) and/or their functional groups(b1) have already been incorporated into the binders (A),self-crosslinking applies.

[0034] Suitable complementary functional groups (a2) of the binders (A)are preferably thio, amino, hydroxyl, carbamate, allophanate, carboxyland/or (meth)acrylate groups, but especially hydroxyl groups, andsuitable complementary functional groups (b1) are preferably anhydride,carboxyl, epoxy, blocked isocyanate, urethane, methylol, methylol ether,siloxane, amino, hydroxyl and/or beta-hydroxyalkylamide groups, butespecially blocked isocyanate groups.

[0035] Examples of suitable functional groups (a1), which are ionic orare convertible into ionic groups, of the binders (A) are

[0036] (a11) functional groups which can be converted into cations byneutralizing agents and/or quaternizing agents, and/or cationic groups,or

[0037] (a12) functional groups which can be converted into anions byneutralizing agents, and/or anionic groups.

[0038] The binders (A) containing functional groups (a11) are used incathodic electrodeposition coating materials whereas the binders (A)containing functional groups (a12) are employed in anodicelectrodeposition coating materials.

[0039] Examples of suitable functional groups (a11) for use inaccordance with the invention that can be converted into cations byneutralizing agents and/or quaternizing agents are primary, secondary ortertiary amino groups, secondary sulfide groups or tertiary phosphinegroups, especially tertiary amino groups or secondary sulfide groups.

[0040] Examples of suitable cationic groups (a11) for use in accordancewith the invention are primary, secondary, tertiary or quaternaryammonium groups, tertiary sulfonium groups or quaternary phosphoniumgroups, preferably quaternary ammonium groups or quaternary ammoniumgroups, tertiary sulfonium groups, but especially quaternary ammoniumgroups.

[0041] Examples of suitable functional groups (a12) for use inaccordance with the invention that may be converted into anions byneutralizing agents are carboxylic, sulfonic or phosphonic acid groups,especially carboxylic acid groups.

[0042] Examples of suitable anionic groups (a12) for use in accordancewith the invention are carboxylate, sulfonate or phosphonate groups,especially carboxylate groups.

[0043] The selection of the groups (a11) or (a12) should be made so asto rule out the possibility of any disruptive reactions with thefunctional groups (a2) which are able to react with the crosslinkingagents (B). The skilled worker will therefore be able to make theselection in a simple way on the basis of his or her knowledge of theart.

[0044] Examples of suitable neutralizing agents for functional groups(a11) convertible into cations are organic and inorganic acids such assulfuric acid, hydrochloric acid, phosphoric acid, amidosulfonic acid,lactic acid, dimethylolpropionic acid, or citric acid, especially formicacid, acetic acid or lactic acid.

[0045] Examples of suitable neutralizing agents for functional groups(a12) convertible into anions are ammonia, ammonium salts, such as, forexample, ammonium carbonate or ammonium hydrogen carbonate and alsoamines, such as, for example, trimethylamine, triethylamine,tributylamine, dimethylaniline, diethylaniline, triphenylamine,dimethylethanolamine, diethylethanolamine, methyldiethanolamine,triethanolamine and the like.

[0046] In general, the amount of neutralizing agent is chosen so thatfrom 1 to 100 equivalents, preferably from 50 to 90 equivalents, of thefunctional groups (a11) or (a12) of the binder (b1) are neutralized.

[0047] Examples of suitable binders (A) for anodic electrodepositioncoating materials are known from the patent DE 28 24 418 A1. Theycomprise, preferably, polyesters, epoxy resin esters,poly(meth)acrylates, maleate oils or polybutadiene oils having a weightaverage molecular weight of from 300 to 10 000 daltons and an acidnumber of from 35 to 300 mg KOH/g.

[0048] Examples of suitable binders (A) for cathodic electrodepositioncoating materials are known from the patents EP 0 082 291 A1, EP 0 234395 A1, EP 0 227 975 A1, EP 0 178 531 A1, EP 0 333 327, EP 0 310 971 A1,EP 0 456 270 A1, U.S. Pat. No. 3,922,253 A, EP 0 261 385 A1, EP 0 245786 A1, EP 0 414 199 A1, EP 0 476 514 A1, EP 0 817 684 A1, EP 0 639 660A1, EP 0 595 186 A1, DE 41 26 476 A1, WO 98/33835, DE 33 00 570 A1, DE37 38 220 A1, DE 35 18 732 A1 or DE 196 18 379 A1.

[0049] These binders are preferably resins (A) containing primary,secondary, tertiary or quaternary amino or ammonium groups and/ortertiary sulfonium groups and having amine numbers of preferably between20 and 250 mg KOH/g and a weight average molecular weight of preferablyfrom 300 to 10 000 daltons. It is preferred to use amino (meth)acrylateresins, amino epoxy resins, amino epoxy resins having terminal doublebonds, amino epoxy resins having primary and/or secondary hydroxylgroups, amino polyurethane resins, amino-containing polybutadieneresins, or modified epoxy resin-carbon-dioxide-amine reaction products.

[0050] Particularly preferred resins used as binders (A) are modifiedepoxy resins in accordance with WO 98/33835, which are obtainable byreacting an epoxy resin with a mixture of monophenols and diphenols,reacting the resulting product with a polyamine to give an amino epoxyresin, and then reacting the resulting amino epoxy resin in a furtherstage with an organic amine to give the modified epoxy resin (cf. WO98/33835, page 19, line 1 to page 21, line 30).

[0051] In accordance with the invention, it is preferred to use cathodicelectrodeposition coating materials, especially cathodicelectrodeposition coating materials based on the above-described binders(A), and the corresponding electrodeposition coating baths.

[0052] The electrodeposition coating materials preferably comprisecrosslinking agents (B).

[0053] Examples of suitable crosslinking agents (B) whose use ispreferred are blocked organic polyisocyanates, especially blockedso-called paint polyisocyanates, containing blocked isocyanate groupsattached to aliphatic, cycloaliphatic, araliphatic and/or aromaticmoieties.

[0054] For their preparation, preference is given to usingpolyisocyanates having from 2 to 5 isocyanate groups per molecule andhaving viscosities of from 100 to 10 000, preferably from 100 to 5000and in particular from 100 to 2000 mPa·s (at 23° C.). Moreover, thepolyisocyanates may have been hydrophilically or hydrophobicallymodified in a customary and known manner.

[0055] Examples of suitable polyisocyanates are described, for example,in “Methoden der organischen Chemie”, Houben-Weyl, Volume 14/2, 4thedition, Georg Thieme Verlag, Stuttgart 1963, pages 61 to 70, and by W.Siefken, Liebigs Annalen der Chemie, Volume 562, pages 75 to 136.

[0056] Further examples of suitable polyisocyanates are isophoronediisocyanate (i.e.5-isocyanato-1-isocyanatomethyl-1,3,3-trimethylcyclohexane),5-isocyanato-1-(2-isocyanatoeth-1-yl)-1,3,3-trimethylcyclohexane,5-isocyanato-1-(3-isocyanatoeth-1-yl)-1,3,3-trimethylcyclohexane,5-isocyanato-(4-isocyanatobut-1-yl)-1,3,3-trimethylcyclohexane,1-isocyanato-2-(3-isocyanatoprop-1-yl)cyclohexane,1-isocyanato-2-(3-isocyanatoeth-1-yl)cyclohexane,1-isocyanato-2-(4-isocyanatobut-1-yl)cyclohexane,1,2-diisocyanatocyclobutane, 1,3-diisocyanatocyclobutane,1,2-diisocyanatocyclopentane, 1,3-diisocyanatocyclopentane,1,2-diisocyanatocyclohexane, 1,3-diisocyanatocyclohexane,1,4-diisocyanatocyclohexane, dicyclohexylmethane 2,4′-diisocyanate,dicyclohexylmethane 4,4′-diisocyanate, liquid dicyclohexylmethane4,4′-diisocyanate with a trans/trans content of up to 30% by weight,preferably 25% by weight and in particular 20% by weight, obtainable byphosgenating isomer mixtures of bis(4-aminocyclohexyl)methane or byfractionally crystallizing commercially customarybis(4-isocyanatocyclohexyl)methane in accordance with the patents DE 4414 032 A1, GB 1220717 A1, DE 16 18 795 A1 or DE 17 93 785 A1;trimethylene diisocyanate, tetramethylene diisocyanate, pentamethylenediisocyanate, hexamethylene diisocyanate, ethylethylene diisocyanate,trimethylhexane diisocyanate, heptamethylene diisocyanate ordiisocyanates derived from dimeric fatty acids, as marketed under thecommercial designation DDI 1410 by the company Henkel and described inthe patents WO 97/49745 and WO 97/49747, especially2-heptyl-3,4-bis(9-isocyanatononyl)-1-pentylcyclohexane, 1,2-, 1,4- or1,3-bis(isocyanatomethyl)cyclohexane, 1,2-, 1,4- or1,3-bis(2-isocyanatoeth-1-yl)cyclohexane,1,3-bis-(3-isocyanatoprop-1-yl)cyclohexane or 1,2-, 1,4- or1,3-bis(4-isocyanatobut-1-yl)cyclohexane, m-tetramethylxylylenediisocyanate (i.e. 1,3-bis(2-isocyanatoprop-2-yl)benzene or tolylenediisocyanate.

[0057] Examples of suitable polyisocyanate adducts areisocyanato-functional polyurethane prepolymers which are preparable byreacting polyols with an excess of polyisocyanates and are preferably oflow viscosity. It is also possible to use polyisocyanates containingisocyanurate, biuret, allophanate, iminooxadiazinedione, urethane, urea,carbodiimide and/or uretdione groups. Polyisocyanates containingurethane groups, for example, are obtained by reacting some of theisocyanate groups with polyols, such as trimethylolpropane and glycerol,for example.

[0058] Very particular preference is given to the use of mixtures ofpolyisocyanate adducts containing uretdione and/or isocyanurate and/orallophanate groups and based on hexamethylene diisocyanate, as areformed by catalytic oligomerization of hexamethylene diisocyanate usingappropriate catalysts. Moreover, the polyisocyanate constituent maycomprise any desired mixtures of the free polyisocyanates exemplified.

[0059] Examples of suitable blocking agents for preparing the blockedpolyisocyanates (B) are the blocking agents known from the U.S. Pat. No.4,444,954 A or U.S. Pat. No. 5,972,189 A, such as

[0060] i) phenols such as phenol, cresol, xylenol, nitrophenol,chlorophenol, ethylphenol, t-butylphenol, hydroxybenzoic acid, esters ofthis acid, or 2,5-di-tert-butyl-4-hydroxytoluene;

[0061] ii) lactams, such as ε-caprolactam, δ-valerolactam,γ-butyrolactam or β-propiolactam;

[0062] iii) active methylenic compounds, such as diethyl malonate,dimethyl malonate, ethyl or methyl acetoacetate, or acetylacetone;

[0063] iv) alcohols such as methanol, ethanol, n-propanol, isopropanol,n-butanol, isobutanol, t-butanol, n-amyl alcohol, t-amyl alcohol, laurylalcohol, ethylene glycol monomethyl ether, ethylene glycol monoethylether, ethylene glycol monopropyl ester, ethylene glycol monobutylether, diethylene glycol monomethyl ether, diethylene glycol monoethylether, diethylene glycol monopropyl ether, diethylene glycol monobutylether, propylene glycol monomethyl ether, methoxymethanol,2-(hydroxyethoxy)phenol, 2-(hydroxypropoxy)phenol, glycolic acid,glycolic esters, lactic acid, lactic esters, methylolurea,methylolmelamine, diacetone alcohol, ethylenechlorohydrin,ethylenebromohydrin, 1,3-dichloro-2-propanol, 1,4-cyclohexyldimethanolor acetocyanohydrin;

[0064] v) mercaptans such as butyl mercaptan, hexyl mercaptan, t-butylmercaptan, t-dodecyl mercaptan, 2-mercaptobenzothiazole, thiophenol,methylthiophenol or ethylthiophenol;

[0065] vi) acid amides such as acetoanilide, acetoanisidinamide,acrylamide, methacrylamide, acetamide, stearamide or benzamide;

[0066] vii) imides such as succinimide, phthalimide or maleimide;

[0067] viii)amines such as diphenylamine, phenylnaphthylamine, xylidine,N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine,dibutylamine or butylphenylamine;

[0068] ix) imidazoles such as imidazole or 2-ethylimidazole;

[0069] x) ureas such as urea, thiourea, ethyleneurea, ethylenethioureaor 1,3-diphenylurea;

[0070] xi) carbamates such as phenyl N-phenylcarbamate or 2-oxazolidone;

[0071] xii) imines such as ethyleneimine;

[0072] xiii)oximes such as acetone oxime, formaldoxime, acetaldoxime,acetoxime, methyl ethyl ketoxime, diisobutyl ketoxime, diacetylmonoxime, benzophenone oxime or chlorohexanone oximes;

[0073] xiv) salts of sulfurous acid such as sodium bisulfite orpotassium bisulfite;

[0074] xv) hydroxamic esters such as benzyl methacrylohydroxamate (BMH)or allyl methacrylohydroxamate; or

[0075] xvi) substituted pyrazoles, imidazoles or triazoles; and also

[0076] mixtures of these blocking agents, especially dimethylpyrazoleand triazoles, malonic esters and acetoacetic esters, dimethylpyrazoleand succinimide, or butyl diglycol and trimethylolpropane.

[0077] Further examples of suitable crosslinking agents (B) are allknown aliphatic and/or cycloaliphatic and/or aromatic polyepoxides,based for example on bisphenol A or bisphenol F. Examples of suitablepolyepoxides also include the polyepoxides obtainable commercially underthe designations Epikote® from Shell, Denacol® from Nagase ChemicalsLtd., Japan, such as, for example, Denacol EX-411 (pentaerythritolpolyglycidyl ether), Denacol EX-321 (trimethylolpropane polyglycidylether), Denacol EX-512 (polyglycerol polyglycidyl ether), and DenacolEX-521 (polyglycerol polyglycidyl ether).

[0078] As crosslinking agents (B) it is also possible to usetris(alkoxycarbonylamino)triazines (TACT) of the general formula

[0079] Examples of suitable tris (alkoxycarbonylamino) triazines (B) aredescribed in the patents U.S. Pat. No. 4,939,213 A, U.S. Pat. No.5,084,541 A, and EP 0 624 577 A1. Use is made in particular of thetris(methoxy-, tris(butoxy- and/ortris(2-ethylhexoxycarbonylamino)triazines.

[0080] The methyl/butyl mixed esters, the butyl 2-ethylhexyl mixedesters, and the butyl esters are of advantage. They have the advantageover the straight methyl ester of better solubility in polymer melts,and also have less of a tendency to crystallize out.

[0081] Further examples of suitable crosslinking agents (B) are aminoresins, examples being melamine resins, guanamine resins, benzoguanamineresins or urea resins. Also suitable are the customary and known aminoresins some of whose methylol and/or methoxymethyl groups have beendefunctionalized by means of carbamate or allophanate groups.Crosslinking agents of this kind are described in the patents U.S. Pat.No. 4,710,542 A and EP 0 245 700 B1 and also in the article by B. Singhand coworkers, “Carbamylmethylated Melamines, Novel Cross-linkers forthe Coatings Industry” in Advanced Organic Coatings Science andTechnology Series, 1991, Volume 13, pages 193 to 207.

[0082] Further examples of suitable crosslinking agents (B) arebeta-hydroxyalkylamides such asN,N,N′,N′-tetrakis(2-hydroxyethyl)adipamide orN,N,N′,N′-tetrakis-(2-hydroxypropyl)adipamide.

[0083] Further examples of suitable crosslinking agents (B) arecompounds containing on average at least two groups capable oftransesterification, examples being reaction products of malonicdiesters and polyisocyanates or of esters and partial esters ofpolyhydric alcohols of malonic acid with monoisocyanates, as describedin the European patent EP 0 596 460 A1.

[0084] The amount of the crosslinking agents (B) in theelectrodeposition coating material may vary widely and is guided inparticular, firstly, by the functionality of the crosslinking agents (B)and, secondly, by the number of crosslinking functional groups (a) whichare present in the binder (A), and also by the target crosslinkingdensity. The skilled worker is therefore able to determine the amount ofthe crosslinking agents (B) on the basis of his or her general knowledgein the art, possibly with the aid of simple rangefinding experiments.Advantageously, the crosslinking agent (B) is present in theelectrodeposition coating material in an amount of from 5 to 60, withparticular preference from 10 to 50, and in particular from 15 to 45% byweight, based in each case on the solids content of the coating materialof the invention. It is further advisable here, to choose the amounts ofcrosslinking agent (B) and binder (A) such that in the electrodepositioncoating material the ratio of functional groups (b1) in the crosslinkingagent (B) to functional groups (a2) in the binder (A) is from 2:1 to1:2, preferably from 1.5:1 to 1:1.5, with particular preference from1.2:1 to 1:1.2, and in particular from 1.1:1 to 1:1.1.

[0085] The electrodeposition coating material may comprise customarycoating additives (C) in effective amounts.

[0086] Examples of suitable additives (C) for pigmentedelectrodeposition coating materials are

[0087] organic and/or inorganic pigments, anticorrosion pigments and/orfillers such as calcium sulfate, barium sulfate, silicates such as talcor kaolin, silicas, oxides such as aluminum hydroxide or magnesiumhydroxide, nanoparticles, organic fillers such as textile fibers,cellulose fibers, polyethylene fibers, titanium dioxide, carbon black,iron oxide, zinc phosphate or lead silicate; these additives may also beincorporated into the electrodeposition coating materials of theinvention by way of pigment pastes.

[0088] These additives (C) are of course not present in the unpigmentedelectrodeposition coating materials.

[0089] Examples of additives (C) which may be present both in pigmentedand in unpigmented electrodeposition coating materials are

[0090] free-radical scavengers;

[0091] organic corrosion inhibitors;

[0092] crosslinking catalysts such as organic and inorganic salts andcomplexes of tin, lead, antimony, bismuth, iron or manganese, preferablyorganic salts and complexes of bismuth and of tin, especially bismuthlactate, citrate, ethylhexanoate or dimethylol-propionate, dibutyltinoxide or dibutyltin dilaurate;

[0093] slip additives;

[0094] polymerization inhibitors;

[0095] defoamers;

[0096] emulsifiers, especially nonionic emulsifiers such as alkoxylatedalkanols and polyols, phenols and alkylphenols or anionic emulsifierssuch as alkali metal salts or ammonium salts of alkanecarboxylic acids,alkanesulfonic acids, and sulfo acids of alkoxylated alkanols andpolyols, phenols and alkylphenols;

[0097] wetting agents such as siloxanes, fluorine compounds, carboxylicmonoesters, phosphoric esters, polyacrylic acids and their copolymers,or polyurethanes;

[0098] adhesion promoters;

[0099] leveling agents;

[0100] film-forming auxiliaries such as cellulose derivatives;

[0101] flame retardants;

[0102] organic solvents;

[0103] low molecular mass, oligomeric and high molecular mass reactivediluents which are able to participate in the thermal crosslinking,especially polyols such as tricyclodecanedimethanol, dendrimericpolyols, hyperbranched polyesters, polyols based on metathesis oligomersor on branched alkanes having more than eight carbon atoms in themolecule;

[0104] anticrater agents;

[0105] Further examples of suitable coatings additives are described inthe textbook “Lackadditive” [Additives for coatings] by Johan Bieleman,Wiley-VCH, Weinheim, N.Y., 1998.

[0106] The above-described crosslinking agents (B) and/or theabove-described additives (C) may also be present in the surfacersdescribed below.

[0107] In accordance with the invention, lead-free cathodicelectrodeposition coating materials afford particular advantages and aretherefore used with preference.

[0108] For the preparation of the electrodeposition coating film, veryparticular preference is given to the use of an electrodepositioncoating material which comprises

[0109] (A) at least one modified epoxy resin as binder, preparable byreacting epoxy resin with a mixture of monophenols and diphenols,reacting the resulting product with a polyamine to give an amino epoxyresin, and then reacting the resulting amino epoxy resin in a furtherstage with a further polymamine to give the modified epoxy resin; and

[0110] (B) at least one blocked polyisocyanate as crosslinking agent.

[0111] The predrying temperature in step b) in that case is from 70° C.to 120° C., preferably from 80° C. to 100° C.

[0112] It is a very important advantage of the process of the inventionthat it is possible to use even those of the above-describedelectrodeposition coating materials which suffer a baking loss of morethan 10% on curing, without the occurrence of the problems mentioned atthe outset.

[0113] Examples of suitable surfacers or antistonechip primers are knownfrom the patents U.S. Pat. No. 4,537,926 A1, EP 0 529 335 A1, EP 0 595186 A1, EP 0 639 660 A1, DE 44 38 504 A1, DE 43 37 961 A1, WO 89/10387,U.S. Pat. No. 4,450,200 A1, U.S. Pat. No. 4,614,683 A1, WO 94/26827 orEP 0 788 523 B1. The surfacers in this case may be present asconventional—i.e., solventborne—or as aqueous coating materials. It isalso possible to use powder coating materials or powder slurry coatingmaterials.

[0114] Preference is given to the use of aqueous surfacers.

[0115] It is preferred to use aqueous surfacers comprising as binder awater dilutable polyurethane resin. Particular preference is given toaqueous surfacers based on water dilutable polyurethane resinsobtainable by reacting with one another polyester polyols and/orpolyether polyols, polyisocyanates, compounds containing at least oneisocyanate-reactive group and at least one (potentially) anionic groupin the molecule, and also, if desired, compounds containing hydroxyland/or amino groups. To prepare the surfacer, the polyurethane resin isneutralized at least partly and dispersed in water. The dispersion isthen made up with pigments and crosslinking agents (cf., for example,the European patent EP 0 788 523 B1, page 5, lines 1 to 29).

[0116] The function of the surfacer or of the coating produced from itis to even out disruptive unevennesses (in the micrometer range) on thesurface of a substrate, so that the surface of the substrate need not besubjected to a leveling pretreatment prior to the application of acoating. This is also done using the comparatively high coat thicknessof the surfacer. It additionally serves to absorb and dissipatemechanical energy, as occurs on stone impact.

[0117] The multicoat system produced by the process of the invention maybe used per se (2-coat system) for the abovementioned end uses. However,it may also be overcoated with a clearcoat or solid-color topcoat,giving a 3-coat system which offers an economical alternative tocomparatively expensive coatings. For demanding applications where aparticularly good appearance is critical, the multicoat system producedby the process of the invention may further be coated with a colorand/or effect basecoat/clearcoat system, preferably by the wet-on-wettechnique (4-coat system).

[0118] These processes for producing multicoat systems are being usedincreasingly in industrial coating for articles of all types inindustrial and private use. Examples of such articles are radiators,wheelrims or hubcaps. However, they may also be used to coat automobilebodies.

[0119] The invention additionally relates to a method of determining thepredrying temperature of the electrodeposition coating material in aprocess for producing a multicoat system of the type specified at theoutset. In the method, a determination is made of the temperature T_(p)at which a viscoelastic property of the as yet unbaked electrodepositioncoating material exhibits an extreme value, and the predryingtemperature is chosen to be the same as or above, preferably from 0° C.to 35° C. and more preferably from 5° C. to 25° C. above, thistemperature T_(p).

[0120] The finding underlying this method is that the primary factor inthe predrying of an electrodeposition coating material in the course ofthe wet-on-wet application of an electrodeposition coating material anda surfacer is not the evaporation of the solvents, but instead that itis important to exceed the temperature at which internal changes of theelectrodeposition coating material take place which manifest themselvesin an extreme value of a viscoelastic property.

[0121] The viscoelastic property of the electrodeposition coatingmaterial that is considered in this case is the loss factor tanδ. Animprovement in the coating result at predrying temperatures above themaximum of the loss factor tanδ has been demonstrated in numerousexperiments.

[0122] The DMTA is a widely known measurement method for determining theviscoelastic properties of coatings and is described, for example, inMurayama, T., Dynamic Mechanical Analysis of Polymeric Materials,Elsevier, N.Y., 1978, pages 299 to 329 and Loren W. Hill, Journal ofCoatings Technology, Vol. 64, No. 808, May 1992, pages 31 to 33. Theprocess conditions during the measurement of tanδ by means of DMTA aredescribed in detail by Th. Frey, K. -H. Groβe-Brinkhaus, U. Röckrath, inCure Monitoring of Thermoset Coatings, Progress In Organic Coatings 27(1996) 59-66, or in DE 44 09 715 A1.

[0123] As well as in DMTA, such a signal may also be found in the courseof viscosity measurements of wet cathodic electrodeposition coatingfilms and the corresponding evaluation in accordance with tanδ=G″/G′(cf. cf. T. Dirking, K. -H. Groβe-Brinkhaus, RheologischeCharakterisierung von Elektrotauchlacken während des Einbrennvorgangs:Korrelation von Viskositätswerten mit Kantenschutzergebnissen[Rheological characterization of electrodeposition coating materialsduring the baking procedure: correlation of viscosity values with edgeprotection results], Fatipec XXIII, 1996, Brussels, Belgium, pages B-260to B-271). Since these measurements are generally taken after predryingat from 100° C. to 110° C., it is evident here too that this is acharacteristic of the cathodic electrodeposition coating system.

[0124] The particular advantages of the process of the invention arenot, however, restricted to the combination of electrodeposition coatingand surfacer coating but instead extend to the coatings lying above themas well. Accordingly, the clearcoats, solid-color topcoats or colorand/or effect basecoat/clearcoat systems produced on top of them have animproved surface appearance (appearance of the overall system includingclearcoat). This is manifested, for example, in the values of alongwave/shortwave wavescan (light reflection) which gives a value forthe amount of scattered light. The flow of the coating material is alsoimproved.

[0125] Moreover, an improvement may be observed in the antistonechipproperties. In particular, the flaking area is smaller and there isbetter adhesion to the substrate.

INVENTIVE EXAMPLES

[0126] 1. Preparation of a Crosslinking Agent (C1) for anElectrodeposition Coating Material

[0127] A reactor is charged under nitrogen with 10 462 parts of isomersand oligomers of higher functionality based on 4,4′-diphenylmethanediisocyanate, having an NCO equivalent weight of 135 g/eq (Lupranat®M20S from BASF AG; NCO functionality approx. 2.7; 2,2′- and2,4′-diphenylmethane diisocyanate content less than 5%). 20 parts ofdibutyltin dilaurate are added and 9626 parts of butyl diglycol areadded dropwise at a rate such that the product temperature remains below60° C. Following the addition, the temperature is held at 60° C. for afurther 60 minutes and an NCO equivalent weight of 1120 g/eq is measured(based on solid fractions). After the product has been diluted in 7737parts of methyl isobutyl ketone, and 24 parts of dibutyltin dilauratehave been added, 867 parts of melted trimethylolpropane are added at arate such that the product temperature does not exceed 100° C. Followingthe addition, reaction is continued for 60 minutes. The mixture iscooled to 65° C. and simultaneously diluted with 963 parts of n-butanoland 300 parts of methyl isobutyl ketone. The solids content is 70.1% (1h at 130° C.).

[0128] 2. Preparation of a Precursor (AC1) for an ElectrodepositionCoating Binder

[0129] The water of reaction is removed at from 110° C. to 140° C. froma 70% strength solution of diethylenetriamine in methyl isobutyl ketone.The solution is subsequently diluted with methyl isobutyl ketone untilit has an amine equivalent weight of 131 g/eq.

[0130] 3. Preparation of an Aqueous Electrodeposition Coating BinderDispersion (D1)

[0131] In a reactor fitted with a stirrer, reflux condenser, internalthermometer and inert gas inlet, 6150 parts of epoxy resin based onbisphenol A, having an epoxy equivalent weight (EEW) of 188, are heatedto 125° C. under nitrogen together with 1400 parts of bisphenol A, 355parts of dodecylphenol, 470 parts of p-cresol and 441 parts of xylene,and the mixture is stirred for 10 minutes. It is subsequently heated to130° C. and 23 parts of N,N-dilmethylbenzylamine are added. The reactionmixture is held at this temperature until the EEW has reached a value of880 g/eq.

[0132] Then a mixture of 7097 parts of the crosslinking agent C1 and 90parts of the additive K 2000 (polyether from Byk Chemie, Germany) isadded and the mixture is held at 100° C. Half an hour later, 211 partsof butyl glycol and 1210 parts of isobutanol are added.

[0133] Immediately thereafter, a mixture of 467 parts of the precursorAC1 (from step 2) and 520 parts of methylethanolamine is introduced intothe reactor and the mixture is conditioned to 100° C. After a furtherhalf-hour, the temperature is raised to 105° C. and 159 parts ofN,N-dimethylaminopropylamine are added.

[0134] 75 minutes after the addition of the amine, 903 parts ofPlastilit® 3060 (propylene glycol compound from BASF AG) are added andthe mixture is diluted with 522 parts of propylene glycol phenyl ether(mixture of 1-phenoxy-2-propanol and 2-phenoxy-1-propanol, from BASF AG)and at the same time cooled rapidly to 95° C.

[0135] After 10 minutes, 14 821 parts of the reaction mixture aretransferred to a dispersing vessel. There, 474 parts of lactic acid (88%strength in water), dissolved in 7061 parts of deionized water, areadded with stirring. The mixture is subsequently homogenized for 20minutes before being diluted further with an additional 12 600 parts ofdeionized water in small portions.

[0136] The volatile solvents are removed by distillation under reducedpressure and then replaced by an equal volume of deionized water.

[0137] The dispersion D1 possesses the following characteristics: Solidscontent: 33.8% (1 h at 130° C.) 29.9% (0.5 h at 180° C.) Base content:0.71 milliequivalents/g solids (130° C.) Acid content: 0.36milliequivalents/g solids (130° C.) pH: 6.3 Particle size: 116 nm (massaverage from photon correlation spectroscopy)

[0138] 4. Preparation of the Aqueous Electrodeposition Binder Dispersion(D2)

[0139] The binder dispersion D2 is prepared in exactly the same way asthe binder dispersion D1 except that, immediately after the dilutionwith propylene glycol phenyl ether, 378 parts of K-KAT 348 (bismuth2-ethylhexanoate from King Industries, USA) are admixed to the organicstage with stirring. After cooling, 14 821 parts of the reaction mixtureare dispersed in exactly the same way as dispersion D1.

[0140] The dispersion D2 possesses the following characteristics: Solidscontent: 33.9% (1 h at 130° C.) 30.1% (0.5 h at 180° C.) Base content:0.74 milliequivalents/g solids (130° C.) Acid content: 0.48milliequivalents/g solids (130° C.) pH: 5.9 Particle size: 189 nm (massaverage from photon correlation spectroscopy)

[0141] 5. Preparation of a Crosslinking Agent (C2) for anElectrodeposition Coating Material (In Analogy to WO 98/33835)

[0142] A reactor is charged under nitrogen with 1084 g parts of isomersand oligomers of higher functionality based on 4,4′-diphenylmethanediisocyanate, having an NCO equivalent weight of 135 g/eq (Lupranat®M20S from BASF AG; NCO functionality approx. 2.7; 2,2′- and2,4′-diphenylmethane diisocyanate content less than 5%). 2 g ofdibutyltin dilaurate are added and 1314 g of butyl diglycol are addeddropwise at a rate such that the product temperature remains below 70°C. It may be necessary to carry out cooling. After the end of addition,the temperature is held at 70° C. for a further 120 minutes.

[0143] The solids content is >97% (1 h at 130° C.).

[0144] 6. Preparation of the Aqueous Electrodeposition Coating BinderDispersion (D3) (In Analogy to WO 98/33835, Example 2.3)

[0145] In a reactor fitted with a stirrer, reflux condenser, internalthermometer and inert gas inlet, 1128 parts of epoxy resin based onbisphenol A, having an epoxy equivalent weight (EEW) of 188, 94 parts byweight of phenol and 228 parts of bisphenol A are heated to 127° C.under nitrogen. With stirring, 1.5 parts of triphenylphosphine areadded, whereupon an exothermic reaction occurs and the temperature risesto 160° C. The mixture is allowed to cool again to 130° C. and theepoxide content is monitored. The EEW of 532 indicates that >98 of thephenolic OH groups have reacted. The subsequent addition of 157 parts ofPluriol P 600 (polypropylene glycol MW 600, BASF) is accompanied bycooling. When the mixture reaches 95° C., 115.5 parts of diethanolamineare added, whereupon an exothermic reaction occurs and the temperaturerises to 115° C. After a further 40 minutes at 115° C., 61.2 parts ofN,N-dimethylaminopropylamine are added. Following a short exothermicperiod (T_(max) 140° C.), the mixture is left to react further for 2hours at 130° C. until the viscosity remains constant. Then 97.6 partsof butyl glycol and 812 parts of the crosslinking agent (C2) are added,with simultaneous cooling, and the mixture is discharged at 105° C.

[0146] 2400 parts of the still-hot mixture are dispersed immediatelywith intensive stirring in a mixture of 2173 parts of fully deionizedwater and 49.3 parts of glacial acetic acid. 53 parts of K-KAT 348(bismuth 2-ethylhexanoate from King Industries, USA) are added to thismixture, and the resulting mixture is homogenized briefly and thendiluted with a further 752 parts of fully deionized water, after whichit is filtered through plate filter K 900 (from Seitz). The dispersionD3 has the following characteristics: Solids content: 45% (1 h at 130°C.) 40.1% (0.5 h at 180° C.) Base content: 0.82 milliequivalents/gsolids (130° C.) Acid content: 0.42 milliequivalents/g solids (130° C.)pH: 6.1 Particle size: 129 nm (mass average from photon correlationspectroscopy)

[0147] 7. Preparation of an Epoxy-Amine Adduct Solution That Is Used(E1)

[0148] In accordance with Example 1.3 of EP 0 505 445 B1, anorganic-aqueous solution of an epoxy-amine adduct is prepared byreacting, in a first stage, 2598 parts of bisphenol A diglycidyl ether(epoxy equivalent weight (EEW): 188 g/eq), 787 parts of bisphenol A, 603nparts of dodecylphenol and 206 parts of butyl glycol in the presence of4 parts of triphenylphosphine at 130° C. to an EEW of 865 g/eq. Coolingis accompanied by dilution with 849 parts of butyl glycol and 1534 partsof D.E.R.®732 (polypropylene glycol glycidyl ether from DOW Chemical),and reaction is continued at 90° C. with 266 parts of2,2′-aminoethoxyethanol and 212 parts of N,N-dimethylaminopropylamine.After 2 hours, the viscosity of the resin solution is constant (5.3dPa·s; 40% strength in Solvenon® PM (methoxypropanol from BASF AG); coneand plate viscometer at 23° C.). The product is diluted with 1512 partsof butyl glycol and the base groups are partly neutralized with 201parts of glacial acetic acid; the product is diluted further with 1228parts of deionized water and discharged.

[0149] This gives a 60% strength aqueous-organic resin solution whose10% dilution has a pH of 6.0.

[0150] 8. Preparation of a Pigment Paste

[0151] First of all, a premix is formed from 277 parts of water and 250parts of the epoxy-amine adduct described in step 5. Then 5 parts ofcarbon black, 60 parts of Extender ASP 200, 351 parts of titaniumdioxide TI-PURE® R 900 (from DuPont) and 54 parts of dibutyltin oxide(Fascat 4203 from Elf-Atochem) are added and mixed for 30 minutes undera high-speed dissolver stirring mechanism. The mixture is subsequentlydispersed in a stirred laboratory mill for from 1 to 1.5 h to a Hegmanfineness of 12 μm and is adjusted if necessary to the desired processingviscosity using further water.

[0152] Solids content: 60% (0.5 h, 180° C.)

[0153] 9. Preparation of the Electrodeposition Coating Materials

[0154] The electrodeposition coating binder dispersions D1-D3 and, ifappropriate, the pigment paste (step 8) were used to prepare thefollowing electrodeposition coating materials: ETL1 ETL2 ETL3 DispersionD1 2771 — — Dispersion D2 — 2492 — Dispersion D3 — — 1871 Pigment paste 313 DI water 1916 2508 3129

[0155] The electrodeposition coating materials obtained in this way havea solids content of approximately 20% in the case of the pigmentedsystem ETL1 and 15% in the case of the clearcoats ETL2-3.

[0156] The application conditions (deposition voltage, depositiontemperature) were chosen so that, following bath aging of a minimum of24 h and baking (15 minutes at a panel temperature of 180° C.), smoothfilms with a thickness of approximately 20 μm were obtained on steelpanels (e.g., Bo 26 W 42 OC) which had not been given a passivatingrinse.

[0157] The process conditions for the determination of T_(p) of theelectrodeposition coating materials by means of DMTA were as follows(cf. Th. Frey, K. -H. Groβe-Brinkhaus, U. Röckrath, Cure Monitoring ofThermoset Coatings, Progress in Organic Coatings 27 (1996) 59-66, or inDE 44 09 715 A1): 1. Preparation: deposition of electrodepositioncoating material on a carbon fiber mesh (Sigratex from Sigri) 2.Instrument: DMA MK IV (from Rheometric Scientific) 3. Conditions:tensile mode, amplitude 0.2%, frequency 1 Hz 4. Temperature ramp: 1°C./min from room temperature to 200° C.

[0158] In the DMTA, a sudden sharp change in the storage modulus E′ andthe loss factor tanδ occurred at about 145° C., and can be attributed tothe beginning of the crosslinking reaction. It was also possible to seethat below this crosslinking temperature the loss factor tanδ shows amaximum (peak) at a temperature T_(p) of about 90° C. As was found byfurther investigations, this maximum was not attributable to a glasstransition in the cathodic electrodeposition coating material underconsideration.

[0159] During a repeat scan of the temperature range from 20° C. to 100°C. with a ten-minute holding time at 100° C. it was found that, evenafter several runthroughs a peak at about 80° C. in the loss factor tanδoccurred in each case. Only a slight shift in the signal toward highertemperatures during the temperature cycles was observed, which pointedto evaporation or drying. The peak itself, however, was retained, sothat the basis for this signal could not lie in drying phenomena.

[0160] For the inventive experiments, the panels were not baked butinstead only predried in a forced air oven for 10 minutes at 80° C. orat 100° C. The temperatures were chosen because the electrodepositioncoating films deposited, as described above, had a maximum of a lossfactor tanδ at T_(p)>80° C.

[0161] 10. Preparation of Water Dilutable Polyurethane Resins

[0162] The preparation is as in Example 1.1 of EP 0 788 523 B1.

[0163] 11. Preparation of Aqueous Surfacers

[0164] The preparation is as in Example 2.a of EP 0 788 523 B1.

[0165] The aqueous coating material (step 11) is applied with a dry filmthickness of 19 μm to the predried cathodic electrodeposition coatingsand is itself predried at 70° C. Subsequently, cathodicelectrodeposition coating and the aqueous coating material are bakedtogether for 15 minutes at a panel temperature of 180° C.

[0166] For further tests (especially stonechip testing), the panels areovercoated with a commercially customary white basecoat material havinga dry film thickness of 18 μm and with a commercially customarytwo-component clearcoat material having a film thickness of 35-40 μm.These films are baked at 130° C. for 30 minutes.

[0167] In order to determine the overall appearance, a black basecoatmaterial with a film thickness of 14 μm is used instead of the whitebasecoat material.

[0168] The test results of the individual coating systems, as a functionof the drying conditions in particular, are collated in the tablesbelow. TABLE 1 Technological testing Test system: Cathodicelectrodeposition coating wet-on- wet with aqueous surfacer Applicationmethod: A. Predrying of cathodic electrodeposition coating at  80° C.,B. Predrying of cathodic electrodeposition coating at 100° C., thenconjoint baking for 15′ at 180° C. (panel temp.) and white topcoatsystem. ETL1 ETL2 ETL3 T_(p): 89° C. ETL1 T_(p): 86° C. ETL2 T_(p): 82°C. ETL3 Application A B A B A B VDA stone 2 1-2 1-2 1-2 2 1-2chipping^(a)) MB ball 7 6 9 5 40 5.5 bombardment flaking^(b)) MB ball 31 4 2 5 1 bombardment degree of rusting^(b))

[0169] The results of Table 1 underscore the significant improvementwhich occurs in the stonechip resistance when the electrodepositioncoating films are predried above the temperature T_(p). TABLE 2 Effectof application on the surface appearance Test system: Cathodicelectrodeposition coating wet-on- wet with aqueous coating materialApplication method: A. Predrying of cathodic electrodeposition coatingat  80° C., B. Predrying of cathodic electrodeposition coating at 100°C., then conjoint baking for 15′ at 180° C. (panel temp.) and blacktopcoat system. ETL1 ETL1 ETL2 ETL2 ETL3 ETL3 Application A B A B A BWavescan Lw^(c)) 5 4 7.4 3.2 17 3.0 Wavescan Sw^(c)) 42 28.1 35 22.2 6028.7

[0170] The results of Table 1 underscore the significant improvementwhich occurs in the appearance when the electrodeposition coating filmsare predried above the temperature T_(p).

What is claimed is:
 1. A process for producing a multicoat system on asubstrate, in which a) an electrodeposition coating film is deposited onthe substrate, b) the electrodeposition coating film is predried byheating to a predrying temperature for a predetermined period, c) a coatof a surfacer is applied to the electrodeposition coating film, and d)the electrodeposition coating film and the coat of the surfacer arebaked together at elevated temperatures, characterized in that thepredrying temperature in step b) is equal to the temperature (T_(p)) orlies above the temperature (T_(p)) at which the loss factor tanδ, whichis the quotient formed from the loss modulus E″ and the storage modulusE′, of the unbaked electrodeposition coating material shows a maximum.2. The process as claimed in claim 1, characterized in that thepredrying temperature is from 0° C. to 35° C., preferably from 5° C. to25° C., above the temperature (T_(p)) at which the loss factor tanδ ofthe unbaked electrodeposition coating material shows a maximum.
 3. Theprocess as claimed in claim 1 or 2, characterized in that theelectrodeposition coating film is prepared using an electrodepositioncoating material comprising (A) at least one modified epoxy resin asbinder, preparable by reacting epoxy resin with a mixture of monophenolsand diphenols, reacting the resulting product with a polyamine to givean amino epoxy resin, and then reacting the resulting amino epoxy resinin a further stage with an organic amine to give the modified epoxyresin; and (B) at least one blocked polyisocyanate as crosslinkingagent.
 4. The process as claimed in claim 3, characterized in that thepredrying temperature in step b) is from 70° C. to 120° C., preferablyfrom 80° C. to 100° C.
 5. The process as claimed in one of claims 1 to4, characterized in that the prescribed period of predrying in step b)is from 1 to 60 minutes, preferably from 5 to 15 minutes.
 6. The processas claimed in one of claims 1 to 5, characterized in that the thicknessof the fully cured electrodeposition coating film is from 10 μm to 30μm, preferably from 15 μm to 20 μm.
 7. The process as claimed in one ofclaims 1 to 6, characterized in that the thickness of the fully curedsurfacer coat is from 10 μm to 60 μm.
 8. The process as claimed in oneof claims 1 to 7, characterized in that the substrate is cooled to theambient temperature before the application of the surfacer.
 9. Theprocess as claimed in one of claims 1 to 8, characterized in that theelectrodeposition coating material is a cathodically depositable coatingmaterial.
 10. The process as claimed in one of claims 1 to 9,characterized in that the surfacer is an aqueous coating material. 11.The process as claimed in claim 10, characterized in that the surfacercomprises a water-soluble polyurethane resin as binder.
 12. The processas claimed in one of claims 1 to 11, characterized in that the surfacercoat forms the topmost coat of the multicoat system (2-coat system) oris overcoated with a clearcoat or solid-color topcoat (3-coat system) orwith a color and/or effect basecoat/clearcoat system (4-coat system).13. The use of multicoat systems produced with the aid of the process asclaimed in one of claims 1 to 12 in automobile coating and in industrialcoating.
 14. The use as claimed in claim 13, characterized in that theindustrial coating embraces the coating of radiators, wheelrims andhubcaps.
 15. A method of determining the predrying temperature of theelectrodeposition coating material in a process for producing amulticoat system comprising the steps of a) depositing anelectrodeposition coating film on the substrate, b) predrying theelectrodeposition coating film by heating at a predrying temperature fora prescribed period, c) applying a coat of a surfacer to theelectrodeposition coating film, and d) conjointly baking theelectrodeposition coating film and the coat of the surfacer at elevatedtemperatures, characterized in that the temperature (T_(p)) isdetermined at which the loss factor tans of the electrodepositioncoating material, which is the quotient formed from the loss modulus E″and the storage modulus E′, in the as yet unbaked state has an extremevalue, and in that the predrying temperature is chosen to be equal to,or from 0° C. to 35° C. above this temperature (T_(p)).
 16. The methodas claimed in claim 15, characterized in that the predrying temperatureis chosen to be 5° C. to 25° C. above the temperature (T_(p)) at whichthe loss factor tanδ of the electrodeposition coating material in the asyet unbaked state has an extreme value.
 17. The method as claimed inclaim 15 or 16, characterized in that the loss factor tanδ is determinedwith the aid of dynamic mechanical thermoanalysis (DMTA).